Which antenna type is characterized for having its elements arranged in a straight line, useful in applications needing directional transmission?

The choice of collinear antennas is appropriate here because they are specifically designed with their elements arranged in a straight line. This configuration allows for directional transmission, making collinear antennas particularly effective when there is a need to focus the signal in a specific direction.

Collinear antennas have overlapping radiation patterns that enhance gain in the direction of the target while suppressing signals in other directions. This directional capability is vital in applications such as point-to-point communications or in systems requiring precise beam alignment.

In contrast, other antenna types mentioned, such as rhombic antennas, have a different arrangement and are primarily used for long-distance communication, often providing a wider radiation pattern rather than a strong directionality. End-fire antennas also have a linear arrangement, but they radiate energy primarily in the direction of the elements and require specific feeding techniques for optimal performance. Doublet antennas are generally configured as dipoles, which also focus in certain directions but are not strictly linear and are typically not as directional as collinear antennas.

Therefore, the distinctive linear arrangement and enhanced directivity are critical features that make the collinear antenna the correct choice for applications needing directional transmission.